Conventional imaging is based on measuring the relative scattered field from a remote object over a known two-dimensional (2D) space and then using various imaging algorithms (e.g., synthetic aperture, holographical, reconstruction techniques, and the like) to detect properties of the object. Different reconstruction techniques may be used to produce an image (e.g., dielectric distribution and geometrical features) of the object. Although there are many different approaches for imaging at microwave and millimeter wave frequencies, each imaging approach has a limiting feature (e.g., hardware or reconstruction algorithms). For example, microwave three-dimensional (3D) imaging methods that use synthetic aperture radar (SAR) imaging techniques typically require a coherent vector (e.g., magnitude and phase) measurement obtained over a 2D surface.
In a typical system, a vector reflectometer is scanned on a one-dimensional (1D) or 2D plane to acquire data (e.g., phase) representative of a sample under test. A computer controls the scanning and data acquisition processes and passes the acquired data through a SAR algorithm to produce a 2D or 3D image of the sample under test.
Three-dimensional imaging requires a wideband signal, especially for obtaining high range resolution. But, designing a radio frequency (RF) receiver with a high dynamic range at very high frequencies (e.g., microwave and millimeter wave) to provide wideband vector measurement is technically difficult and costly. To obtain an image, the reflectometer must be calibrated such that all the phase measurements are referenced to the reflectometer's antenna aperture; or the reflectometer's phase center, which changes as a function of frequency, for all frequencies within the band.
For instance, a homodyne full quadrature reflectometer provides an in-phase and a quadrature signal proportional to the real and imaginary parts of the reflected wave, respectively. An advantage of a homodyne full quadrature reflectometer is the receiver can be calibrated to remove the effect of unwanted signals, such as those reflected from various connections and to make the measurements phase referenced to the reflectometer's antenna aperture. For wideband frequency operation, the design of this reflectometer must be drastically altered towards a heterodyning scheme, which is technically difficult and costly due to the requirement of having multiple sources, mixers, amplifiers, and filters.
Another conventional reflectometer based on frequency modulated continuous wave (FM-CW) radars does not provide a vector measurement. Thus, it cannot be readily calibrated (e.g., phase referenced) to the antenna aperture, for imaging purposes.
Therefore, what is needed is a wideband imaging system that can be phase referenced to its antenna aperture.